Lignins are aromatic polymers contained in many plants including trees or gramineous plants, and are contained together with celluloses and hemicelluloses in these plants. The above three components are present as a lignocellulose in which the components are complicatedly bonded to each other, in a cell wall of the plants. Therefore, it is not easy to separate the lignocellulose into these components. Also, lignins have a higher reactivity than that of the celluloses and are therefore readily susceptible to condensation reaction upon heating, and finally transformed into a massive substance that is inert and has a poor solubility in solvents.
As the method of separating lignins from the lignocellulose raw material with a high yield, there is known a Kraft cooking method used mainly by paper-making companies (for example, refer to Patent Document 1). Also, studies have been made on a method of disentangling celluloses and lignins from each other by treating the lignocellulose with hydrogen peroxide as an oxidizing agent (for example, refer to Patent Document 2) or by immersing the lignocellulose in an organic solvent under high-temperature and high-pressure conditions (for example, refer to Patent Document 3). In addition, studies have also been made on a method of removing polysaccharides from the lignocellulose raw material using an enzyme to utilize the resulting enzymatic saccharification residue (for example, refer to Patent Document 4).